This invention relates to a transmitting and/or receiving head for a sonic flowmeter for fluids, which operates by the runtime principle and incorporates an enclosure, an ultrasonic transducer that transmits ultrasound signals into the moving fluid and/or receives such signals from the moving fluid, and an ultrasound waveguide by way of which the ultrasound signals are injected into and/or extracted from the moving fluid.
Ultrasound-based flowmeters are being used to a progressively increasing extent for industrial flow-rate measurements of fluids, i.e. moving liquids and gases. Their particular advantage lies in the fact that, much like for instance magneto-inductive flowmeters, they permit xe2x80x9cnon-contactxe2x80x9d flow measurements without requiring any interfering structure in the flow path.
The two key measuring methods used with sonic flowmeters involve the runtime and the Doppler principle, respectively, with the runtime method offering substantially better attainable accuracy than the Doppler approach. It is for that reason that sonic flowmeters employing the runtime method, and especially the differential-runtime method, are generally preferred.
The runtime of an ultrasound signal along the measuring path from the transmitting ultrasonic transducer to the receiving ultrasonic transducer in a given fluid is a function of the sound propagation speed and the flow rate of the fluid (entrainment). This is the underlying operating principle of runtime-based sonic flowmeters. In the differential runtime method, ultrasound signals are transmitted upstream and downstream through the fluid either in alternating fashion or simultaneously. Due to their different propagation rates over the measuring path having geometrically identical length, the upstream and downstream signals arrive at their respective receiving ultrasonic transducers after different runtimes. The runtime difference between the two soundwaves is directly proportional to the flow rate of the moving fluid.
Ultrasound measurements of a flowing medium necessarily require a measuring tube through which passes the fluid to be measured, plus at least one ultrasonic transducer and preferably two ultrasonic transducers axially offset relative to each other in the flow direction, as well as a control circuit and an analytical circuit for determining the runtime of the ultrasound signals. Beyond that, the control and analysis circuitry can derive from the runtime the flow speed and the flow volume or other parameters of the moving fluid.
Given that, due to their high accuracy and operational reliability, sonic flowmeters have been used with such great success and many more potential uses are opening up for these flowmeters. Salient among these are applications involving high-temperature fluids. Utilization in the realm of petroleum and natural-gas extraction is but one example.
High temperatures, however, pose a significant problem for the application of sonic flowmeters. In general, the base component of an ultrasonic transducer is a piezo crystal, which limits conventional flowmeters to a temperature range peaking at 150xc2x0 C. Any higher temperatures will cause the piezo crystals to become failure-prone or totally disfunctional.
The publication JP 61-93914 A, which gave rise to this invention, describes a sonic flowmeter in which the ultrasonic transducer inserts the ultrasound signals into the moving fluid by way of an ultrasound waveguide. This earlier sonic flowmeter attempts to raise the permissible fluid temperature limit by avoiding direct contact between the ultrasonic transducer and the fluid and, instead, incorporating an ultrasound waveguide between the moving fluid and the ultrasonic transducer. That design has certain shortcomings, however, in that, for one, the decay time of the ultrasound signals is very long due to the strong reflections of the ultrasound signals along the boundary surfaces of the ultrasound waveguide, while there is also a substantial risk of ultrasound-signal crosstalk between the ultrasound waveguide and the measuring tube as well as the enclosure. Moreover, at the point of transition between the ultrasound waveguide and the moving fluid, a significant part of the energy of the ultrasound signal emanating from the ultrasonic transducer is retroreflected into the ultrasound waveguide. The rather minimal transmission factor of the ultrasound signal, attributable to the varying impedances of the ultrasound waveguide and the moving fluid, is proportional to the impedance and inversely proportional to the temperature of the fluid. It follows that if, for example, one wants to determine the flow rate of a hot gaseous fluid, the sonic power injected into the gas is too weak in the case of conventional sonic flowmeters to be received by the other ultrasonic transducer, the impedance of the hot gas being very low.
This invention is, therefore, aimed at enhancing and improving the concept of the earlier sonic flowmeters in a way as to avoid these shortcomings, permitting reliable and accurate flow measurements even of hot fluids and especially hot gases.
According to the invention, this is accomplished by means of a specially designed transceiver head basically and essentially characterized in that the head employs as the ultrasound waveguide an elongated sonic finnel offering high thermal conduction resistance. First of all, a sonic funnel focusses the sonic energy so that in terms of a particular cross-sectional plane, a stronger sonic signal can be injected into the fluid. In addition, a suitably dimensioned sonic funnel will ensure a sufficient relative temperature drop between the fluid and the ultrasonic transducer. The thermal conduction resistance of the sonnic funnel is determined using the following equation:
R=∫ymuL[A(y)xcex(y)]xe2x88x921dyxe2x80x83xe2x80x83Equation (1)
where A is the cross-sectional plane, L the length and xcex the thermal conductivity of the sonic funnel. By suitably dimensioning the cross section A, the length L and the thermal conductivity xcex it is possible to produce a sonic funnel with a thermal conduction resistance R sufficiently high to prevent the temperature at the ultrasonic transducer from exceeding the permissible level.
In a preferred embodiment of the transceiver according to this invention, it is desirable to provide the sonic funnel with an at least partly circular, cylindrical funnel sleeve. It is also desirable to close off the end of the sonic funnel facing the fluid with a window through which the ultrasound signals are injected into and/or extracted from the fluid. If, in fact, the sonic funnel is equipped with a funnel sleeve, it will be very desirable to close off the sonic funnel and the funnel sleeve with a window. Optimal injection of the ultrasound signals into the fluid and extraction of the ultrasound signals from the fluid can be ensured preferably by centering the sonic funnel on the window and by dimensioning the diameter and thickness of the window in such fashion that the highest possible oscillation amplitude of the window can be attained.
A particularly preferred embodiment of the transmitting and/or receiving head according to this invention is further characterized in that the funnel sleeve is provided with attenuation so as to dissipate the reflected ultrasound waves in the funnel sleeve. This conversion of the sonic energy into heat loss is further enhanced by positioning an impedance matching element in front of the attenuation element on the side facing the window. This impedance matching element causes the undesirable sonic reflections in the funnel sleeve to be shunted into the attenuator. Using this type of impedance matching can improve the effectiveness of the attenuator by a factor of about 2. Overall, combining attenuation and impedance matching is particularly useful in shortening the decay time of the ultrasound signal and reducing the ultrasound-signal crosstalk between the funnel sleeve and the enclosure.
The transceiver according to this invention works especially well in a sonic flowmeter equipped with two such transceivers as well as control and analysis circuitry, where the said control and analysis circuitry serves to measure the runtime for determining the volumetric flow. This control and analysis circuitry preferably uses the difference between the total runtime of the ultrasound signals between the ultrasonic transducers and the sum of the runtimes of the ultrasound signals in the sonic funnels for determining the volumetric flow.
There are numerous individual possibilities to further enhance and expand the disclosed transmitting and/or receiving head for a sonic flowmeter. Reference to these is made in the dependent patent claims, as well as in the following description of a preferred embodiment explained with the aid of the following drawings.